The mechanical structure and function of loudspeaker drivers of the kind which have an electro-acoustic transducer are well known. A flexible surrounding suspension connects the periphery of the diaphragm to the surrounding frame. A second, smaller flexible suspension element connects the centre portion of the diaphragm to the frame; both suspension elements permit a drive unit, usually a “voice coil”, to move the diaphragm axially in order to radiate sound waves. Ideally, diaphragm motion is directly proportional to the electrical signal that is fed into the voice coil. Due to dimensional and material limitations, however, this is not attainable in practice. One source of such limitations is the suspension itself because, as the diaphragm is displaced axially, unwanted deformation occurs at certain axial displacements.
The surround suspension performs several functions in a loudspeaker driver, namely, (i) keeping the diaphragm (cone) centered; (ii) sealing the loudspeaker driver in an enclosure or baffle and separating air mass in front of and behind of the cone, thereby avoiding cancellation; and (iii) allowing diaphragm (cone) to move back and forth to produce sound. In order to reproduce low frequencies properly, the diaphragm needs to move a substantial distance without becoming unstable. This has led to the use of a “roll” suspension which flexes back and forth as the diaphragm moves axially. A wider roll (longer in transverse cross-section) will permit greater travel of the diaphragm but, for a given frame, increasing the width of the roll surround suspension reduces the effective radiator area of the diaphragm.
The axial displacement permitted by a roll suspension having a semi-circular profile is limited to about 1.2 times its width because, at that point, the roll is fully stretched into a conical surface. A roll suspension having a parabolic or elliptical profile with a height more than half its width will permit a greater displacement than a semi-circular roll suspension having the same width.
Such a parabolic roll surround suspension is disclosed in U.S. Pat. No. 3,997,023 (White). Although such a roll suspension might permit adequate axial movement, however, at certain displacements unwanted deformation of the suspension itself may occur, causing non-linearity and introducing distortion in the output of the loudspeaker. More particularly, it has been found that “wrinkling” occurs when the diaphragm moves inwards. This is caused by compression of the surround material and is most pronounced with taller and/or wider rolls, such as in subwoofers. Such wrinkles produce sound distortion and can even cause the surround material to break. In practice, therefore, total diaphragm displacement or excursion becomes limited by buckling deformation of the roll.
Several solutions have been proposed for unwanted deformation of the roll suspension while maintaining range of travel and linearity. Some propose the use of reinforcing elements, for example radially-oriented ridges at intervals around the surround, as taught in U.S. Pat. No. 6,725,967 (Dikbowicki), or angularly-oriented notches, as taught by U.S. Pat. No. 7,054,459 (Kuze et al.), or varying the thickness or density at intervals of a compressed neoprene surround as taught in US2003/0228027 (Czerwinski). While this may help in reducing wrinkling/buckling, it does not completely eliminate it because there are still sections of the roll where compressive stress concentrates. In addition there is excessive stress concentration around notches or ridges due to relatively abrupt or sharp transitions in geometry. This could even lead to eventual material failure due to fatigue.
U.S. Pat. No. 7,275,620 (Dietrich et al.) discloses a rectangular loudspeaker in which the generally rectangular surround has notches and ridges defining trapezoidal formations at the corners. This is not entirely satisfactory because relatively sharp transitions would still lead to unwanted stress and non-linearity.
U.S. Pat. No. 6,889,796 (Pocock et al.) discloses a surround suspension in which the cross-section of the roll alternates between semi-circular and semi-elliptical, forming convolutions or undulations. In addition, the fillet where the roll meets the diaphragm varies sinusoidally in phase with the undulations. An obvious drawback of such solution is that the excursion is still limited to that of the semicircular portions of the roll. Also stress would occur around the transitions, leading to deformation.
Yet another solution, disclosed in U.S. Pat. No. 7,397,927 (Pircaro et al.), adds angularly—(as opposed to radially-) oriented convolutions to a base profile of the roll. While this might help to reduce wrinkling/buckling, it is not entirely satisfactory because it would tend to introduce torsional stress into the drive system and have an adverse effect upon linearity.
With a view to improving high frequency stability by preventing unwanted deformation in the diaphragm, U.S. Pat. No. 6,697,496 (Frasl) discloses a low profile suspension having pleats along its length. The pleats are divided into three equal 120 degree segments, with the pleats in each segment parallel to each other but oriented at an angle of 120 degrees to those in the other two segments. While this might help to reduce unwanted diaphragm oscillations at higher frequencies, the arrangement would not be entirely satisfactory because its low profile would mitigate against it use for low frequency loudspeakers and stress concentrations would occur between adjacent pleats having different orientations, i.e., endmost pleats of the different segments.
U.S. Pat. Nos. 6,851,513 and 7,174,990 (Stead et al.) disclose a surround suspension whose peak varies in shape around the circumference, either by alternating between a semi-circular cross-section and other conic section that is greater in height, or by varying the radius of the peak sinusoidally along the circumference. Neither option is entirely satisfactory because the uniform sections would tend not to reduce buckling completely and/or relatively sharp transitions between peaks and uniform roll sections would lead to stress and distortion.
U.S. Pat. No. 7,438,155 (Stead et al.) discloses a loudspeaker drive unit similar to those disclosed in their above-mentioned patents. In this case, however, the peak of the roll surround varies sinusoidally around the perimeter while maintaining constant height, but its cross-sectional shape varies. This too would not be entirely satisfactory because the transitions between peaks and troughs would still introduce stress and distortion.
In effect, convolutions transform material compression into bending, which flexible materials are designed to do, but known configurations are not entirely satisfactory because the geometry of the convolutions does not adequately reduce stress at transitions, leading to deformation stress in the suspension material and concomitant distortion in the loudspeaker driver output.